This invention relates to a method and apparatus for accurately sensing and reproducibly controlling the intensity of energy of a gaseous material activated by a source of energy. In its most preferred form, this invention is specially suited for use in the mass production of amorphous (as defined hereinafter) semiconductor devices by deposition processes in which it is helpful to determine and control the energy level to which said gaseous material is subjected.
Glow discharge deposition comprises one method of mass producing amorphous semiconductor devices. It is a process carried out at less than atmospheric pressure, wherein at least one reactant gas, introduced into a sealed deposition chamber, is decomposed under the effects of an electromagnetic field developed in a portion (the decomposition region) of that chamber. Whether the electromagnetic energy is provided by alternating current, direct current, radio frequency, or microwave frequency, it is adapted to excite the atoms and molecules of the reactant gas(es), causing the products of the gaseous decomposition to be deposited upon a substrate located within the deposition chamber. The present invention is directed toward the novel concept of accurately sensing and reproducibly controlling the intensity of the electromagnetic energy introduced into the deposition chamber for decomposing the reactant gas(es).
It is to be noted that, as used herein: (1) the term "reactant gas" defines the process gas or gases from which the material (in the preferred embodiment, semiconductor material) deposited upon the substrate is derived, whether the reactant gas(es) comprise a single species, or a mixture of species, and may include an inert carrier gas admixed therewith; (2) the term "glow discharge deposition" includes all deposition processes wherein the reactant gas(es) is decomposed by electromagnetic energy regardless of whether (a) a visible glow is developed, or (b) an additional source(s) of energy, such as thermal energy, is utilized in conjunction with the electromagnetic energy; and (3) the term "amorphous" includes all alloys or materials which have long range disorder, although they may have short or intermediate range order or even contain, at times, crystalline inclusions.
Recently, considerable efforts have been made to develop systems for depositing amorphous semiconductor materials, each of which can encompass relatively large areas and which can be doped to form p-type and n-type materials for the production of p-i-n type photovoltaic devices which are, in operation, substantially equivalent to their crystalline counterparts.
It is now possible to prepare amorphous silicon alloys by glow discharge deposition or vacuum deposition techniques. The amorphous silicon alloys so prepared posses (1) acceptable concentrations of localized states in the energy gaps thereof, and (2) high quality electronic properties. Such techniques are fully described in U.S. Pat. No. 4,226,898, entitled Amorphous Semiconductors Equivalent To Crystalline Semiconductors, issued to Stanford R. Ovshinsky and Arun Madan on Oct. 7, 1980; U.S. Application Ser. No. 423,424 of Stanford R. Ovshinsky, David D. Allred, Lee Walter, and Stephen J. Hudgens entitled Method Of Making Amorphous Semiconductor Alloys And Devices Using Microwave Energy; and U.S. Pat. No. 4,217,374, of Stanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980, also entitled Amorphous Semiconductor Equivalent To Crystalline Semiconductors. As disclosed in these patents, fluorine introduced into the amorphous silicon semiconductor layers operates to substantially reduce the density of the localized states therein.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was described at least as early as 1955 by E. D. Jackson in U.S. Pat. No. 2,949,498 issued Aug. 16, 1960 . The multiple cell structures therein discussed utilized p-n junction crystalline semiconductor devices. Essentially the concept employed different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc). Further, by definition, a tandem cell device has two or more cells with the light directed serially through each cell. In the first cell a large band gap material absorbs only the short wavelength light, while in subsequent cells smaller band gap materials absorb the longer wavelengths of light which pass through the first cell. The overall open circuit voltage in a tandem cell is therefore the sum of the open circuit voltage of each cell, while the short circuit current thereof is maintained substantially constant.
Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, amorphous silicon alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Continuous processing systems of this kind are disclosed, for example, in pending patent applications: Ser. No. 151,301, filed May 19, 1980, for A Method Of Making P-Doped Silicon Films And Devices Made Therefrom; Ser. No. 244,386, filed Mar. 16, 1981, for Continuous Systems For Depositing Amorphous Semiconductor Material; Ser. No. 240,493, filed Mar. 16, 1981, for Continuous Amorphous Solar Cell Production System; Ser. No. 306,146, filed Sept. 28, 1981, for Multiple Chamber Deposition And Isolation System And Method; and Ser. No. 359,852, filed Mar. 19, 1982, for Method And Apparatus for Continuously Producing Tandem Amorphous Photovoltaic Cells. As disclosed in these applications, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific semiconductor material. In making a solar cell of p-i-n type configurations, the first chamber is dedicated for depositing a p-type semiconductor alloy, the second chamber is dedicated for depositing an intrinsic amorphous semiconductor alloy, and the third chamber is dedicated for depositing an n-type semiconductor alloy. Since each deposited semiconductor alloy, and especially the intrinsic semiconductor alloy, must be of high purity, the deposition environment in the intrinsic deposition chamber is isolated from the doping constituents within the other chambers to prevent the back diffusion of doping constituents into the intrinsic chamber.
In addition to such factors as the isolation of the reactant gas(es) in adjacent deposition chambers of the deposition apparatus, all parameters influencing the disassociation and recombination (hereinafter referred to as "decomposition") of those gas(es) must be closely controlled in order to reproducibly manufacture high quality photovoltaic devices. Since the semiconductor layers are formed by the decomposition of the reactant gases under the influence of an electromagnetic field, it follows that the electrical, chemical and optical properties of those layers are directly related to the parameters of the decomposition process. More specifically, even small variations in the amount of electromagnetic energy delivered to the decomposition region of a decomposition chamber for disassociating and recombining the reactant gas(es), can correspondingly alter (1) the chemical composition of the semiconductor material deposited upon the substrate; (2) the rate of deposition of the semiconductor material onto the substrate; (3) the electrical and optical properties exhibited by the deposited semiconductor material; and (4) make the reproducibility of uniform semiconductor materials virtually impossible. The importance of reproducibility in the production of semiconductor devices cannot be emphasized too strongly. Since, as previously mentioned, even minor changes in the compositional properties of a semiconductor material can result in material changes in its electronic and optical properties, (such as band-gap, density of states, and photoconductivity), the electromagnetic field must be carefully controlled in order to control said compositional properties of the semiconductor material. More particularly, the (1) strength of the electromagnetic field in which the reactant gases are disassociated and recombined, and (2) length of time which the reactant gases are subjected to that electromagnetic energy determines the chemical bonding and composition of the semiconductor material deposited onto the substrate. If the strength or intensity of the electromagnetic energy delivered to the decomposition region of a deposition chamber is not kept constant from one day's production of semiconductor material to the next day's production, the electrical and optical properties of that material will vary from day-to-day. However, the problem is still more difficult. It has been determined that the electromagnetic energy introduced into the decomposition region often varies with (1) time or (2) changes in other operational parameters, thereby making it impossible to reproduce results even within the same run. It should be quite apparent that such results are intolerable, not only from a manufacturing standpoint, but from a research and development standpoint as well. In order to improve the quality of photovoltaic devices, it is essential that laboratory results be reproducible for succeeding experiments and for a reduction of those experimental achievements and results to a production mode of operation.
Accordingly, there exists a need for a method of and apparatus adapted to accurately monitor and control the intensity of electromagnetic energy employed to decompose reactant gas(es) in glow discharge deposition apparatus, thereby reproducibly controlling the composition and deposition rate of the semiconductor material deposited onto the substrate. As alluded to hereinabove, initial attempts at achieving this control merely consisted of setting the source of electromagnetic energy, such as a radio frequency generator, at a constant preselected value and assuming the level of electromagnetic energy delivered from that source to the decomposition region of the deposition apparatus would remain constant. This assumption proved incorrect and led to the development of the monitoring and control system disclosed herein.
Simply setting the source of energy to deliver a constant level of electromagnetic energy to the decomposition region of a deposition chamber fails to provide a constant level of power actually acting to decompose the reactant gas(es), since factors such as the coupling of the electromagnetic energy to those reactant gas(es), heat losses, impedance matching, temperature differentials, etc. are not taken into account. Of the aforementioned factors, it appears that the impedance tuning is the most critical. As the deposition system operates, the various components thereof may heat up or cool down, thereby disrupting the transfer of power through the system. Therefore, the amount of power delivered by the source of electromagnetic energy to the reactant gas(es) is likely to change with time of operation of the apparatus, even though the energy source is set to deliver a constant level of power to the decomposition region. Accordingly, the simplistic approach of setting the source of electromagnetic energy to a preselected constant value in order to sense and control the intensity of electromagnetic energy delivered to the decomposition region of a glow dischage deposition system, is not sufficiently accurate to provide a reproducibly controllable level of power.
It is to fill this need for accurately monitoring and reproducibly controlling the intensity of electromagnetic energy in the decomposition region actually decomposing the reactant gas(es) flowing therethrough that the apparatus and method of the instant invention was developed. More precisely, the method and apparatus of the instant invention directly senses the intensity of electromagnetic energy in the decompositon region of glow discharge deposition apparatus by detecting the emission of radiant energy from the excited reactant gas(es) in that decomposition region. A signal indicative of the intensity of the electromagnetic energy actually emitted by the reactant gas(es) is then generated. That signal is utilized in a closed, automatic control loop to assure that the electromagnetic energy acting to decompose the reaction gas(es) remains at a preselected value despite variations in other operating parameters.
The many objects and advantages of the present invention will become clear from the drawings, the detailed description of the invention and the claims which follow.